Molecular imprinting, also referred to as templating, has been used for chiral separations and involves arranging polymerizable functional monomers around a print molecule. This is achieved either by utilizing non-covalent interactions such as hydrogen bonds, ion-pair interactions, etc. (non-covalent imprinting), or by reversible covalent inter-actions (covalent imprinting) between the print molecule and the functional monomers. The resulting complexes are then incorporated by polymerization into a highly cross-linked macroporous polymer matrix. Extraction of the print molecule leaves sites in the polymer with specific shape and functional groups complementary to the original print molecule. Mosbach, K., Trends in Biochemical Sciences, Vol. 7, pp. 92-96, 1994; Wulff, G., Trends in Biotechnology, Vol. 11, pp. 85-87, 1993; and Andersson, et al., Molecular Interactions in Bioseparations (Ngo. T. T. ed.), pp. 383-394.
Different racemic compounds have been resolved via molecular imprinting, i.e., "amino-acid derivatives", see Andersson, et al., Molecular Interactions in Bioseparations (Ngo T. T. ed.), Plenum Press, pp. 383-394, 1993; "drugs", Fischer, et al., J. Am. Chem. Soc., 113, pp. 9358-9360, 1991; Kempe, et al., J. Chromatogr., Vol. 664, pp. 276-279, 1994; and "sugars", Wulff, et al., J. Org. Chem., Vol. 56, pp. 395-400, 1991; Mayes, et al., Anal. Biochem., Vol. 222, pp. 483-488, 1994. Baseline resolution has been achieved in many cases.
An advantage of molecularly imprinted polymers, in contrast to other chiral stationary phases, is the predictable order of elution of enantiomers. Imprintable supports have been prepared from bulk polymerization techniques, using a porogenic solvent to create a block of macroporous polymer. However, bulk polymerization supports must be crushed, ground and sieved to produce appropriate particle sizes for use in separatory columns and analytical protocols. For example, in chromatographic evaluations, polymer particles smaller than 25 .mu.m are generally used. However, from the bulk polymerization process the grinding process used to provide these smaller particles from the bulk polymerization process is unsatisfactory. Grinding produces irregularly shaped particles and an excessive and undesirable quantities of "fines." Typically less than 50 percent (50%) of the ground polymer is recovered as useable particles. Irregular particles generally give less efficient column packing for chromatography and often prove troublesome in process scale-up. Hence, uniformly shaped particles, e.g. beaded polymers, would be preferable in most cases. The grinding process also requires an additional treating step to remove the fines, i.e., sedimentation. This is costly and time consuming. The bulk polymerization and necessary grinding process makes this prior art technique labor intensive, wasteful and unacceptable.
Attempts have been made to use suspension and dispersion polymerization techniques for producing beads from acrylic monomers which can contain imprinted molecules. In principle these suspension and dispersion polymerization techniques should offer an alternative to bulk polymerization. However, existing suspension and dispersion techniques are not satisfactory because water or a highly polar organic solvent (e.g. an alcohol) is used as the continuous phase for the relatively hydrophobic monomers. These solvents are incompatible with most covalent and non-covalent imprinting mixtures due to the competition between solvent and functional monomers for specific interaction with the print molecule. Since suspension polymerization techniques use the solvent in large molar excess, the solvents saturate the monomer phase and drastically reduce the number and strength of the interactions between functional monomers and print molecules. In addition, because of the high solubility of acidic monomers in water, random copolymerization of monomers and cross-linker is probably not achieved. Water soluble print molecules are also lost due to partitioning into the aqueous phase. Not unexpectedly, attempts to make molecularly imprinted polymer beads by suspension polymerization in water have led to only very poor recognition. Damen, et al., J. Am. Chem. Soc., Vol. 102, pp. 3265-3267, 1980; Braun, et al., Chemiker-Zeitung, Vol. 108, pp. 255-257, 1984; Bystrom, et al., J. Am. Chem. Soc., Vol. 115, pp. 2081-2083, 1993. With stable covalent or metal chelate bonds between functional monomers and print molecules prior to polymerization, it may be possible to use aqueous conditions.
Attempts have also been made to produce composite beaded particles by imprinting in the pore network of performed beaded silica, Norrlow, et al., J. Chromatogr., Vol. 299, pp. 29-41, 1984; Wulff, et al., Reactive Polymers, Vol. 3, pp. 261-2757, 1985 or TRIM. However, the preparation requires careful handling and the volume of imprinted polymer per unit column is inevitably reduced by the beads themselves.
Sellergren, B., J. Chromatogr., Vol. 673, pp. 133-141, 1994 and Sellergren, B., Anal. Chem., Vol. 66, pp. 1578-1582, 1994, report the use of dispersion polymerization in a polar solvent mixture for molecular imprinting. The process produces random precipitates rather than regular beads. Acceptable results were only achieved for highly charged print molecules, presumably due to the presence of competing solvent effects.
Thus, a need exists for a method that produces beaded polymers containing molecular imprints that is simple and reproducible, does not compromise the quality of the imprints obtained and eliminates the need for grinding and sieving equipment. A need also exists for a molecular imprinted polymer bead that is uniform.